Alterations in the mechanics of respiration in unanesthetized guinea pigs are used to assess irritant response. Current work has indicated that the respiratory response to a sub-cutaneous injection of 0.125 mg/kg histamine is greater in animals which have been exposed 2 hours earlier to ozone (0.1 to 0.8 ppm) than in animals which received an exposure to air. Continuing studies will examine the effect of ozone pre-exposure on the response to histamine given as a sub-micrometer sized aerosol, and on the response to irritant aerosols such as ammonium sulfate or zinc sulfate. Measurements of static as well as dynamic compliance will be made to assess more precisely the effects in the peripheral portion of the lung. Histopathological studies will be made on the lungs of ozone exposed animals to see if histological changes are observed which would help to explain the observed hyper-reactivity following ozone exposure.